2-digit numbers less than 91 which are 1 less than a multiple of 10

{19, 29, ... , 79, 89}

Step-by-step explanation:

Multiples of 10 are {10, 20, 30, 40, ... , 90}

Numbers which are each 1 less than these multiples of 10 would be:

                               {9, 19, 29, ... , 79, 89}.

Note that the last number, 89, is less than 91.

We must omit 9 from this set, since 9 is not a 2-digit number.  Then our final list is {19, 29, ... , 79, 89}.

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